Glyphosate resistance has become an increasing worry, not just for common waterhemp control, but for managing other annual weeds like giant and common ragweed, concurs Jeff Gunsolus, University of Minnesota Extension weed scientist. “These weeds are out-crossers, meaning that they pollinate over great distances, which gives them more genetic diversity and makes selection for resistance occur faster,” he says. “That’s particularly true for common waterhemp, which has been confirmed to be herbicide resistant to both glyphosate and ALS inhibitor herbicides in Minnesota and several other Corn Belt states.”

Weed flushes in late summer also make common waterhemp challenging to control, Gunsolus says. “To help control that, you need to have a soybean variety that provides a good canopy to shade out the weeds,” he advises.

Waterhemp shows up along rivers and flood plains in western Kentucky but is not yet widespread across the state, says J. D. Green, University of Kentucky Extension weed scientist.

“In Kentucky, we plant both full-season and double-crop soybeans, and corn/wheat/soybeans is a common crop rotation,” Green notes. “That three-crop rotation has helped us with resistant management up to this point by allowing farmers to rotate different herbicides for each crop.”

Glyphosate resistance has become a problem with marestail, the number-one troublesome weed in Kentucky soybeans, Green says. “As farmers have started their shift to using more glyphosate in corn, the tendency has been to back off the rates for preplant or preemergence corn herbicides, which predisposes more weeds to survive the application,” he says. “Yet famers need to use the full, recommended rates of the soil residual herbicides to get as much benefit from them as possible and not just rely on glyphosate to clean up the mess. A heavy reliance on glyphosate for your main weed control program is only going to lead to problems later.”

Marestail is fairly widespread in no-till systems in Kentucky, Green says. “It’s also glyphosate resistant in many fields scattered across the state,” he adds.

Because marestail seeds are windblown, farmers often have difficulty controlling it after the crop has emerged, Green notes. “So we typically recommend using 2,4-D as part of the  burndown program and to include a preplant soil residual herbicide to aid with control,” he adds.

Marestail problems are also common in Illinois, Hager says. “It’s our number-two soybean weed of concern, due mainly to the resistance it has developed to glyphosate, particularly in the southern third of the state,” he says.

“Horseweed/marestail thrives better under a no-till cropping system compared to other tillage systems. So the use of some tillage before planting can be helpful to manage the problem.”